1. Field of the Present Invention
The present invention relates to an improved process for preparing alkali metal aluminum phosphate granules useful as a leavening acid which provides for a cleaner reactor, easier granulation and drying, increased production rates and a decreased heating requirement in the milling stage while providing a product of increased density, lower dustiness and which is substantially nonhygroscopic.
2. Prior Art
Crystalline sodium aluminum phosphate (or SALP) was first disclosed in U.S. Pat. No. 2,550,490 and an early baking powder composition incorporating SALP was disclosed in U.S. Pat. No. 2,550,491. U.S. Pat. No. 2,550,490 specifically discloses a SALP with a Na:Al:PO.sub.4 ratio of 1:3:8. Since that time, several modifications of sodium aluminum phosphate have been developed which give different reactivities and performance characteristics. These include a dehydrated SALP, U.S. Pat. No. 2,957,750; a 3:3:8 SALP, U.S. 3,223,479; a 3:2:8 SALP; U.S. Pat. No. 3,501,314, a 2:3:6 SALP; U.S. Pat. No. 3,574,536, an amorphous SALP; U.S. Pat. No. 2,995,421, a 3:3:9 SALP; U.S. Pat. No. 3,726,962 and the continuous crystallization of SALP, U.S. Pat. No. 3,311,448.
Sodium aluminum phosphate is a well known leavening agent in the baking industry. It finds use in baking powders, self-rising mixes, preleavened pancake flours and mixes, prepared biscuit mixes, and prepared cake mixes (see U.S. Pat. No. 2,550,491, 3,109,738, 3,041,177, 3,076,178). It is also used as a meat binding agent. Sodium aluminum phosphate, basic, is useful as a melt controlling additive in cheese.
Sodium aluminum phosphate has several inherent deficiencies, the most serious of which is dusting and hygroscopicity. Sodium aluminum phosphate dust is very light and rapidly permeates the air in food processing plants, creating cleaning and sanitation problems and unsatisfactory working conditions for the employees. Additionally, the finely divided particles of sodium aluminum phosphate do not flow easily.
Sodium aluminum phosphate is also an inherently hygroscopic material which will absorb a large quantity of atmospheric moisture, usually about 28-29% by weight. Originally produced, SALP is a dry, white crystalline product. If permitted to stand exposed in a hot, humid atmosphere, it rapidly absorbs moisture, first forming water droplets or caking at the surface, then becoming what may be termed as a viscous semi-fluid. Commercially, this phenomenon is minimized somewhat by the use of sealed, air-tight containers.
Several proposals have been made in the past for improving the physical handling properties of sodium aluminum phosphate, particularly directed to improving flow characteristics and dust properties. These approaches have generally been directed to agglomerating or pelletizing the SALP as disclosed, for example, in U.S. Pat. No. 3,620,972 which utilizes water as an agglomerating medium. Other methods have involved the employment of various binders such as sugars and crystallizing syrups. Still other approaches utilize the addition of small amounts of colloidal SiO.sub.2 or tricalcium phosphate to the SALP as flow conditioners to improve physical handling properties.
In connection with SALP 3:2:8 disclosed in U.S. Pat. No. 3,501,314, it is known to dry blend a flow conditioner with dry SALP crystals. The flow conditioner can be any alkali or alkaline earth metal phosphate, calcium hydroxide or aluminum oxide. It is taught that the dry flow conditioner particles are adhered to dry SALP particles as a dry coating. The flow conditioners are taught to increase flow-ability and reduce hygropscopicity. However, flow conditioners in general are known to only improve handling characteristics to a slight degree.
U.S. Pat. No. 3,255,073 to Blanch et al describes a potassium modified sodium aluminum acid phosphate having decreased hygroscopicity. This result is accomplished by modifying the original sodium aluminum phosphate molecule with the introduction of potassium. The potassium is explained as replacing hydrogen atoms in the crystalline lattice of sodium aluminum phosphate. This improved potassium modified sodium aluminum phosphate is described as having hygropscopic properties wherein it does not increase in weight by more than about 20%, preferably not more than about 10% of its original weight during continued exposure at 35.degree. C. and 75% relative humidity for 140 hours.
An improvement over U.S. Pat. No. 3,205,073 is U.S. Pat. No. 3,411,872 to Post et al which attempts to improve the flow characteristics of Blanch et al's potassium modified sodium aluminum phosphate by incorporating the potassium ions in a solvent suspension of an alkanol.
A further improvement over U.S. Pat. No. 3,205,773 is disclosed in U.S. application Ser. No. 671,769, filed Mar. 30, 1976 by R. Benjamin et al. In that application, a specific ratio of sodium and potassium is used to prepare potassium modified SALP. The improved SALP product is characterized by increased density and reduced dusting properties. Among the advantages accured thereby are ease of packaging, use of smaller bags that palletize more easily, decreased hygroscopicity and improved flow characteristics. All of these properties enable better handling, in general, especially under conditions of high humidity.
It is taught in U.S. Pat. No. 2,550,490 that the speed of the gas developing reaction of the sodium aluminum phosphate can be accelerated by the use of an accelerator such as monocalcium phosphate. The monocalcium phosphate may be formed on the surface of the sodium aluminum phosphate crystals by preparing the SALP as usual but omitting an alcohol wash to remove excess phosphoric acid and neutralizing the excess phosphoric acid with hydrated lime. After vigorous agitation, the mixture solidified into small granular lumps. After drying, it was required that the product be milled to provide a dry, non-hydroscopic powder having a neutralizing value of 100.4 and a particle size sufficiently small to be usable in baking. It was tested in the baking of biscuits and found to have baking characteristics equal to that of standard commercial phosphate-alum baking powders.
It has also been found that a calcium treated sodium aluminum phosphate having improved handling characteristics can be prepared by contacting a slurry of sodium aluminum phosphate having an excess of phosphoric acid with a calcium compound reactable with the acid followed by granulating the calcium treated product while drying under such conditions that a majority of the granulated particles are less than 840 micron (through 20 mesh) and at least 90% less than 2000 micron (U.S. application Ser. No. 899,445 which is a continuation U.S. application Ser. No. 703,872, now abandoned). The entire disclosures of these applications (Ser. Nos. 671,769, 703,872, and 899,455) are hereby incorporated by reference. There is provided granulated complex aluminum phosphate granules with a calcium rich, theoretically, a calcium phosphate, outer surface. This process can also be included as part of the process for preparing potassium modified SALP such as described in U.S. application Ser. No. 671,769 above. Since this process requires a reaction between phosphoric acid and a reactive calcium compound such as calcium hydroxide, the stoichiometry of the reaction must be such that an excess of phosphoric acid is provided for. Further, the product contains calcium phosphate which might not be desirable at all times. Also, the product must be granulated while drying. Any beneficial effect is lost if the product is extensively ground.
Various unsuccessful attempts have been made to dry blend sodium aluminum phosphate and calcium sulfate. The dry blended composition tended to cake and blind sifting screens in the processing equipment of the user.